# Spin Coupling in Symmetric and Asymmetric Allyl and Phenalenyl Diradicals Bridged by an Inverted Singlet–Triplet System

## TL;DR

This paper explores how organic diradicals with inverted singlet–triplet units can control spin interactions through optical means.

## Contribution

The study introduces new diradical systems with spin–optical functionality based on 1,3-diazete motifs.

## Key findings

- Diradicals with 1,3-diazete bridges show degenerate singlet and triplet ground states.
- Torsional flexibility enhances spin–orbit coupling and intersystem crossing rates.
- All studied diradicals enable population transfer from singlet to triplet states under ambient conditions.

## Abstract

Organic diradicals
bridged by inverted singlet–triplet (InveST)
units have recently emerged as promising molecular platforms for spin–optical
functionality, enabling optical control of spin–spin interactions
within a fully organic framework. Here, we study a series of symmetric
and asymmetric InveST-bridged diradicals based on the smallest InveST
motif, 1,3-diazete, functionalized with allyl and phenalenyl radical
units. Within the Pariser–Parr–Pople (PPP) framework,
these systems possess disjoint diradical ground states with degenerate
singlet and triplet levels, while population of the bridge LUMO triggers
finite exchange interactions that stabilize the triplet excited state.
Torsional flexibility at the bridge–radical junctions plays
a key role in triggering spin–orbit coupling, thereby promoting
intersystem crossing between the lowest excited singlet and triplet
states. Through evaluation of intersystem crossing and reverse intersystem
crossing rates using a vibronic model, we find that all three diradicals
support net population transfer from the singlet to the triplet manifold
under ambient conditions.